The present invention relates to a printing drum of a stencil printing machine.
As one example of stencil printing machines, a printing machine equipped with a multi-porous cylindrical printing drum rotatably supported on the central axis has been known, in which a perforated stencil sheet is wrapped around the printing drum, and ink is supplied from inside the printing drum to the outside of the printing drum, thus performing printing on printing paper pressed against the stencil wrapped around the outer peripheral surface of the printing drum.
In the stencil printing machine with the basic construction described above, one having more concretely the following mechanism has been known. That is, a base member thereof is formed by connecting, by a transverse bar, two annular members disposed at a specific space on a common axis, and a flexible multi-porous sheet is wrapped in a cylindrical form around nearly the entire outside surface of the base member, thus constituting the printing drum of a cylindrical shape. The flexible multi-porous sheet is fastened at the leading edge part to one edge of the transverse bar and then, after being wrapped in a cylindrical form around the base member, is elastically fixed at the tail edge part to the other edge of the transverse bar through a spring member. Therefore, the flexible multi-porous sheet is so wrapped and freely seated on the annular members as to be slidable in relation to the surface of the annular members along the direction of rotation of the printing drum.
Inside the printing drum are mounted an inner pressure roller and an ink supply device for supplying ink to the surface of the inner pressure roller. When the printing drum is rotated to perform printing, the flexible multi-porous sheet is expanded outwardly in a radial direction, to rotate the inner pressure roller along the inner peripheral surface of the flexible multi-porous sheet, thus supplying ink via the flexible multi-porous sheet and the stencil. Outside the printing drum a back press roller is mounted very closely to, but not in contact with, the printing drum, so that printing is done by transferring ink to printing paper inserted between the printing drum and the back press roller. The printing drum of the above-described constitution has been disclosed for example in Japanese Patent Application No. Hei 1-47029.
According to the stencil printing machine described above, printing is performed by radially outwardly expanding a part of the printing drum while rotating the inner pressure roller along the inner peripheral surface of the flexible multi-porous sheet, and by transferring ink to the printing paper inserted between the expanded part and the back press roller. The tail edge of the flexible multi-porous sheet outwardly expanded by the inner pressure roller is pulled backwardly by the spring member in the direction of rotation of the printing drum along the outer peripheral surface of the annular section.
However, even when the flexible multi-porous sheet is freely seated on the outer peripheral surface of the annular members, considerable frictional resistance occurs in the circumferential direction of the annular members due to contact therebetween, preventing the inner pressure roller from smoothly pushing the flexible multi-porous sheet outwardly at the start of printing.
A frictional force in the circumferential direction of the annular members between the flexible multi-porous sheet and the annular members acts to prevent the inner pressure roller from pushing the flexible multi-porous sheet outwardly, resulting in an insufficient printing pressure applied to paper at the printing starting area of the printing drum which corresponds to the front part of the paper. Accordingly, it results in indistinct printing and further in an increased load to the inner pressure roller and a driving means thereof. Furthermore, ink enters between the flexible multi-porous sheet and the annular members, resulting in nonuniform frictional resistance at both sides as viewed from the transverse bar, an unbalanced contact pressure between the inner pressure roller and the inner peripheral surface of the flexible multi-porous sheet, a nonuniformly transferred amount of ink, and consequently in a nonuniform image. Also, wrinkles will form in the stencil because of different printing pressures applied to paper even at both edges of the stencil corresponding to the circumferential direction of the annular members.